Magnesium alloys are considered to be amongst the advanced materials of the 21st century. Not only are they lightweight (with a density that is approximately two thirds that of aluminium), they have the benefits of high specific strength, stiffness and dent resistance, good damping characteristics and excellent castability. They are particularly attractive for electronics, space and defence applications.
In recent years, the use of wrought magnesium alloy sheet has experienced significant growth in the areas of electronic device enclosures and batteries. Furthermore the United States Council for Automotive Research has initiated research programs to demonstrate the application of wrought magnesium alloy in automobiles. Identified products suitable for manufacture from wrought magnesium alloys include inner panel components, covers, chassis parts and bumper reinforcements.
Typically, a quantity of the alloy is produced into a sheet which can then be shaped to form the desired product using different forming technologies for sheet products, such technologies include blanking, bending, sheet stamping and cup drawing (deep drawing). In conventional production of magnesium alloy sheet via direct-chill (DC) slab casting, the magnesium alloy is supplied as slabs typically 300 mm by 1 m in cross-section and 2 m to 6 m long. These slabs are first homogenized or preheated (for example at 480° C. for AZ31) for several hours and then continuously hot rolled on a reversing hot mill until reduced to about 5 to 6 mm thick. The sheet metal is re-heated at 340° C. before each pass of ˜20% reduction in the final finish mill. New improved production techniques like twin-roll casting (TRC), enables the production of sheets of magnesium alloy direct from molten metal with a thickness less than 10 mm, eliminating the need for much of the repeated rolling, re-heating and sometimes intermediate annealing used in conventional sheet manufacturing methods.
Magnesium, with its hexagonal close packed (HCP) crystal structure, has very limited number of slip systems operable at room temperature for successful rolling. Hence, temperatures between 250° C. to 450° C. are used for rolling a magnesium alloy. Although a wide range of temperatures is used, manufacturers of alloy sheet desire alloys which are suitable for rolling at reasonably low temperatures.
A wrought magnesium alloy that is widely available for sheet metal forming is the alloy designated AZ31B. The nominal composition by weight of this alloy is about three percent aluminium, one percent zinc, controlled and limited amounts of impurities, and the balance magnesium. Common problems that restrict the use of wrought magnesium alloy materials such as AZ31B are the initial cost of the magnesium sheet material associated with existing commercial production techniques and its reduced formability and workability at relatively lower temperatures compared to conventional materials such as aluminium. As such, there is a need to develop new wrought magnesium alloys that have good ductility, formability and workability at lower temperatures and more suitable for commercial use.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.